Nuclear fusion is a process in which lighter nuclei fuse together to form a heavier nucleus with the evolution of large amounts of energy due to the mass defect that occurs.
Some fusion reactions are shown below:
The high temperature required for fusion to start can be made available by nuclear fission process.
This is the principle underlying the hydrogen or thermonuclear bomb. A hydrogen bomb consists of an arrangement of nuclear fission in the centre. It is surrounded by a mixture of deuterium
The nuclear fission provides heat and neutrons. The neutrons are used in converting lithium isotopes into tritium 
and the heat liberated is used for the fusion between 
to start. The fusion reactions are then accompanied by the liberation of large amount of energy. The reactions taking place are:
Though the energy liberated per fusion is smaller that that liberated per fission of U-235 (which is about 200 MeV). Yet the hydrogen bomb is much more powerful than the atomic bomb for the following reasons:
i) Mass per atom of deuterium as well as tritium is much smaller than that of uranium.ii) There are no restrictions of critical mass for fusion process to take place.
Compared with fission reactions, fusion reactions have the advantage that large amounts of highly radioactive nuclides are not obtained as by-products which may pose problem of safe storage.The used up uranium fuel rods from nuclear reactor power plants are very harmful to mankind. It can cause cancer and birth defects. Nuclear power plants have been storing old fuel rods in big swimming pools like concrete tanks. In years to come, power plants may run out of space to store the rods.
